Thesis advisor: Daniel Kirschner / Alzheimer's disease is the leading cause of dementia in the United States. The neurodegenerative condition of this disease correlates with the formation in the brain of plaques consisting of insoluble protein aggregates, termed amyloid. The aggregates are caused by the misfolding of amyloid β, a 40—42 amino acid polypeptide that is naturally occurring in all humans. One approach to preventing the amyloid cytotoxicity is to prevent the formation of plaques altogether. Many types of inhibitors have been tested for their therapeutic value, including substituted peptide strands. In this study, the inhibitory potential of two such peptides was tested: methylated peptides and nitrile-substituted peptides. Aβ(16-22) was used for its fiber-forming properties, and x-ray diffraction and transmission electron microscopy were used to assess the extent of fibrillogenesis. The methylated peptide effectively inhibited fiber formation as previously recorded, and the cyanophenylalanine derivatives did not form fibers. The latter experiment provided insight on the structural and folding properties of Aβ more than its possible inhibitory potential. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Biology. / Discipline: College Honors Program.

The electrostatic field created by the inward pointing dipole moments of an oriented membrane leaflet has never been measured directly, but is thought to have an important influence on membrane function. Here we present the first direct measurement of the membrane dipole field in lipid bicelles using vibrational Stark effect spectroscopy which is based on the sensitivity of a nitrile oscillator’s vibrational frequency to its local electrostatic environment. The nitrile probe was introduced as the artificial amino acid p-cyanophenylalanine (CN-Phe) in four different locations of a α-helical peptide composed of alternating alanine and leucine residues. This peptide was intercalated into bicelles composed of mixtures of the long chain lipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and the short chain lipid 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) formed in two different sizes, 5 nm and 15 nm in radius. Formation of the bicelle above the phase transition temperature of the lipid mixture was confirmed by ³¹P NMR, and the structure of the [alpha]-helix within the bicelle was confirmed by circular dichroic spectroscopy. The absorption energy of the nitrile probe at 4 positions along the helical axis was measured by Fourier transform infrared spectroscopy, from which we estimate the magnitude of the membrane dipole electrostatic field to be -6 MV/cm. Then we successfully manipulated the dipole field in q = 0.5 DMPC/DHPC bicelles by incorporating the small molecule phloretin into the membrane and measured the corresponding ratiometric fluorescence signal of the co-intercalated voltage gated dye di-8-ANEPPS. We measured 0.7 ± 0.2 cm⁻¹ blue shift in absorption energy of the nitrile probe due to the decrease in dipole field caused by phloretin, corresponding to a dipole field of -4.2 MV/cm. This change was essentially identical to what has been estimated through ratiometric fluorescence methods, indicating that VSE spectroscopy will be useful tool for measurement of the biological effects of electrostatic fields in lipid membranes. / text